Answer 6.2

a) Explain the nitrogen rule.

If a compound contains an even number of nitrogen atoms (0, 2, 4, …), its monoisotopic molecular ion will be detected at an even-numbered m/z (integer value).
Vice versa, an odd number of nitrogens (1, 3, 5, …) is indicated by an odd-numbered m/z.

b) Apply the nitrogen rule to the compounds below. Do not calculate molecular weights. Simply fill “even” or “odd” into the second column!

Molecular composition

C3H8

C6H12O2

C6H11NO

C32H25N3O2S

C8H20N2

C7H9N

C3H6BrN

C9H11ClN4O2

C17H32Cl2

Monoisotopic mass is …

even-numbered

even-numbered

odd-numbered

odd-numbered

even-numbered

odd-numbered

odd-numbered

even-numbered

even-numbered

c) Consider a radical containing no nitrogen to be cleaved off from an even-mass molecular ion. Will the mass of the fragment be even- or odd-numbered?

The m/z value of the fragment will be odd-numbered.

d) Consider a molecule containing one nitrogen to be eliminated from a molecular ion containing two nitrogens. Will the mass of the fragment be even or odd-numbered?

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